This invention relates to semiconductor integrated circuits having a subscriber loop interface circuit and, more specifically, the invention relates to semiconductor integrated circuits having a subscriber loop interface circuit including circuitry for internally testing the operation of the subscriber loop interface circuit.
A subscriber loop interface circuit (“SLIC”) is generally employed as an interface between a subscriber such as a residential telephone subscriber and a subscriber network. A SLIC may provide functions such as two-to-four wire conversion, battery feed, line supervision, or common mode rejection.
Generally, a subscriber loop includes a SLIC, a subscriber terminal device (e.g., a telephone), and a pair of subscriber lines connecting the SLIC and the subscriber terminal device.
In one prior art technique for using SLIC's in a subscriber loop, a central office operating as a switching station includes a SLIC for every subscriber being served by the central office. The central office may also include relays for connecting a ringing signal, a tone signal, or an incoming communication to each SLIC.
To maintain a high degree of reliability, the central office may include dedicated test hardware and software for the purpose of testing the integrity of subscriber loops. The dedicated test equipment may include hardware and software for testing the integrity of subscriber loop interface circuits. Typically, the test hardware resides in the central office and is shared for testing several subscriber loops. The central office may include electromechanical relays or solid state switches for connecting each subscriber loop interface circuit to test hardware.
Testing of SLIC's at a central office is usually extensive and includes leakage, capacitance, detection, return loss, and continuity testing. Testing of SLIC's at a central office may also include loop back testing. Typically, a loop back test is conducted by placing a load across the two subscriber line terminals of a SLIC and measuring the resulting current flow. The load may be a 600 ohm resistor. The load is typically placed off-chip to protect the SLIC semiconductor material from heat that may be generated by the load during testing. During this technique for loop back testing, a SLIC is unable to detect a subscriber off-hook signal and, therefore, the subscriber is locked out during testing.
Testing may also be required in other subscriber environments. In loop carrier systems, SLIC's are employed in remote stations. In general, each remote station serves a small number of subscribers and includes a physical or wireless connection to a central office. Loop carrier systems are typically smaller in size and less costly than systems such as residential phone systems. Providing SLIC testing in a remote station may substantially increase the cost and overhead in building a loop carrier system. However, the cost of servicing remotely located stations may also be costly.
Accordingly, it is an object of the present invention to provide a novel semiconductor circuit having a subscriber loop interface circuit comprising a circuit for testing a ring trip detector portion of the subscriber loop interface circuit.
It is another object of the present invention to provide a novel semiconductor circuit having a subscriber loop interface circuit including a circuit for testing an off-hook detect portion of the subscriber loop interface circuit.
It is yet another object of the present invention to provide a novel apparatus in a semiconductor subscriber loop interface circuit for testing both the ring trip detector and the off-hook detect portions of the subscriber loop interface circuit without disabling the capability of the subscriber loop interface circuit to monitor the hook status of the subscriber.
It is still another object of the present invention to provide a novel method for internally testing portions of a subscriber loop interface circuit.
It is a further object of the present invention to provide a novel method and apparatus for simultaneously testing a ring trip detector and an off-hook detect circuit of a subscriber loop interface circuit.
It is yet a further object of the present invention to provide a novel method and apparatus for detecting an off-hook signal during internal testing of an off-hook detect circuit.
It is still a further object of the present invention to provide a novel method and apparatus for internally testing portions of a subscriber loop interface circuit.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.